Recently, in an electrical discharge machining process using an electrode wire for an electrical discharge machining apparatus, the improvement of electrical discharge machining speed is desired from a viewpoint of uplift of productivity, and the electrode wire for the electrical discharge machining apparatus having a covering layer formed around a core wire is proposed.
As a method for manufacturing the electrode wire for the electrode discharge machining apparatus having the covering layer, the same, in which a pipe shaped brass alloy covering layer is formed around a core wire by means of welding, is proposed (Japanese Patent Kokai, 6-47130).
According to the aforementioned method, a brass tape is longitudinally applied around a core wire to form a pipe, a seam formed by butting longitudinal edges of the brass tape is continuously welded to provided a composite wire, and thereafter the composite wire is processed to be reduced in area by means of plural reducing dies. According to the aforementioned method, since a large scale installation is not necessary, manufacturing cost can be reduced. Moreover, since the electrode wire for the electrical discharge machining apparatus of a long length can be continuously manufactured, this method is profitable in productivity.
However, according to the aforementioned method, a seamlike welded composition is formed along the surface of the composite wire in the longitudinal direction, and in case that the welded composition remains intact, the composite wire is defective in workability at the time of area-reduction process, and there is the possibility of occurrence of the breaking of the composite wire, so that area-reduction process of high reduction rate cannot be applied to the composite wire.
Moreover, the welded composition is not dissolved and extant to the last as a part of the electrode wire for an electrical discharge machining apparatus under some unfavorable condition of area-reduction process, and in such a case, electrical discharge machining process becomes unstable at a part of the welded composition.
Accordingly, it is an object of the invention to solve the aforementioned problems and provide a method for manufacturing an electrode wire for an electrical discharge machining apparatus, by which area-reduction process of high reduction rate can be smoothly carried out and an electrode wire for an electrical discharge machining apparatus with stable discharge machining characteristic can be provided.
According to the feature of the invention, a method for manufacturing an electrode wire for an electrical discharge machining apparatus comprises the steps of:
longitudinally applying a brass tape around a core wire formed of Cu or Cu alloy to provide a pipe, PA1 continuously welding a seam formed along the pipe by butting longitudinal edges of the brass tape to provide a composite wire, PA1 applying area-reduction process of reduction rate less than 65% to the pipe in the composite wire by means of a squeezing die, PA1 applying a heat treatment at a temperature higher than a recrystallization one of brass to the composite wire, and PA1 processing the composite wire to be reduced in area by means of plural reducing dies.
Although the maximum reduction rate is 97% in ordinary area-reduction process (drawing process), area-reduction process of reduction rate less than 65% is applied to the brass tape in the composite wire according to the invention as reduction rate necessary for recrystallizing the welded composition in the brass tape in the composite wire. In case that area reduction rate in the ordinary drawing process is applied to the composite wire, the breaking of the composite wire occurs at reduction rate of 65 to 75%, and drawing becomes impossible.
It is well known in the metal engineering that, in order to make the covering layer (the brass tape layer) comprising a welded composition recrystallize by heat treatment, it is necessary to apply moderate plastic working or plastic deformation to the covering layer before heat treatment, and the covering layer is easily recrystallized as the degree of plastic working or plastic deformation is high.
The reason for limiting material of the core wire to Cu or Cu alloy is that the electrical discharge machining characteristic is improved because of high conductivity of the core wire and straightness of the core wire is satisfactory. In case that a steel wire is used as a core wire, the composite wire becomes defective in straightness and is difficult to deal with.
With regard to the diameter of the core wire and the thickness of the brass tape, the diameter of the core wire is selected to be more than 4 mm and the thickness of the brass tape is more than 0.8 mm from a viewpoint of productivity that the diameter of the composite wire is preferably as large as possible.
The reason for that metallic composition of brass is limited to a single phase composition of .alpha. phase or a mixing composition of .alpha. and .beta. phases is that brass containing Zn of high concentration is not suited for area-reduction process (drawing process). On the basis of the aforementioned property of brass, metallic composition of brass is determined so that the drawing process of the composite wire is industrially practicable. Area-reduction process of brass formed of a mixing composition of .alpha. and .beta. phases becomes difficult as concentration of Zn (rate of .beta. phase composition) increases.